Measurements of electron density fluctuations during the ROSE rocket flights

Journal of Aimospherir and Terrestrial Printed in Great Britain.

physics,

Vol. 54. No. 6. pp. 715-723,

0021-9169:92 $S.oO+ .oO Perspxm Press Ltd

1992.

Measurements of electron density fluctuations during the ROSE rocket flights K. SCHLEGEL Max-Planck-Institut

fiir Aeronomie, W-341 1 Katlenburg-Lindau, (Received infinalform

6 May

F.R.G.

1990)

Ah&act-Electron density fluctuations have heen measured with a modified retarding potential analyzer during the four ROSE rocket flights. The instrument is described and the results obtained are explained and discussed. They are generally in agreement with similar earlier results, but also show new features.

1. INTRODU~~ON

The aim of the ROSE project (Rocket and Scatter -._..Experiments) was to study the E-region plasma under the influence of current-driven instabilities. From these instabilities, the modified two stream instability is excited if the drift velocity difference between the electrons and the ions exceeds a threshold close to the ion-acoustic velocity; the gradient-drift instability is excited if, in addition to a finite drift, an electron density gradient is present (SCHLEGEL, 1985). Both instabilities create density fluctuations in the E-region plasma which were studied in this project. Their spatial distribution along the rocket path and their frequency spectrum provide some clues for a better understanding of the instabilities in question, particularly on their saturation level. A suitable instrument to investigate these density structures is a modified retarding potential analyzer (RPA) which will be explained in the next section. Sections 3-5 contain a detailed description of the relevant results ; their discussion in Section 6 concludes the paper. Details of the ROSE project can be found in a special report (ROSE et al., 1990) as well as in the article summarizing the results (ROSEet al., 199 1, this issue). For completeness, the date, time, location and geophysical conditions during the four flights are compiled in Table 1.

2. MEASURING

PRINCIPLE

Usually a retarding potential analyzer (RPA) is designed to measure electron temperature, ion temperature or flux of suprathermal electrons (e.g. KNUDSEN, 1966 ; SPENNER, 1978). If it is operated in the saturation mode, however, the saturation current onto the collector is given by

j = Gen,v,F where G is a geometrical factor, e is the electron charge, n, and v, are the electron density and thermal velocity, respectively, and F is the collecting area. While it is very di~cult to obtain reliable absolute estimates of the electron density, because of the great uncertainty of G, relative current changes AJ!j corresponding to relative density changes Ain,,lpi,are easy to measure. A similar technique with a RPA has been used by MORI et al. (1988). The applied RPA was designed to perform just this task. The measured current in the saturation mode

(1O- I0 to 1O- 6 A) was amplified by a fast electrometer and then sampled each 0.5 ms in order to resolve density ~uctuations up to 1 kHz. Some technical details of the RPA and its sensor are summarized in Table 2. The same method of sensor construction was already successfully applied in several rocket and satellite experiments and has proved to be very reliable (SPENNER and DUMBS, 1974; SPJZNNERet al., 1977 ; SPENNER, 1978). Every 3 s the fast sampling mode was interrupted and the RPA was used in a normal electron temperature mode for 100 ms. The corresponding grid voltages are also included in Table 2. Each RPA on the four ROSE rockets Fl-F4 was equipped with two sensors, one looking in the direction of the rocket motion, the other perpendicular to it.

3, SPECTRUM OF ELECTRON DENSITY FLUCTUATIONS

Figure 1 shows a false colour plot of ~u~tuation intensities vs frequency and time. It is composed of spectra calculated from 200 current samples (at 0.5 ms resolution) via the FFT, yielding a frequency range from 10 to 1000 Hz. The two bands, one during the

716

K.

SCHLEGEL

Table 1. Flight data of the ROSE rockets and geophysical conditions during the flight [from EISCAT data ; KOHL (199 l), this issue]

Flight

Date, start time, location and rocket apogee

Geophysical conditions during the flight relevant to the E-region instabilities

Fl

26 November 1988 1700:00 UT Andoya, Norway 116km

Moderately disturbed E x B drift z 750 m/s, westward Peak electron density (109 km): 2.3 x 10” m -’

F2

5 December 1988 2333:00 UT Andoya, Norway 113 km

Weakly disturbed E x B drift z 500 m/s, slightly south of east Peak electron density (108 km) : 2.5 x 10” m-’

F3

7 February 1989 2336:30 UT Esrange, Kiruna, Sweden 125 km

Weakly disturbed E x B drift z 480 m/s, slightly south of east Peak electron density (100 km) : 4.2 x 10” mm3

F4

9 February 1989 2342:00 UT Esrange, Kiruna, Sweden 123 km

Moderately disturbed E x B drift x 800 m/s, eastward Peak electron density (116 km) : 1.6 x 10” mm’

upleg and one during the downleg, in which the Eregion instabilities are excited, are clearly visible. It is interesting to note that the strongest fluctuations apparently originate at lower heights (at t = 120130 s, corresponding to z 101 km altitude during the upleg and at t = 230-240 s, corresponding to about the same height during the downleg). This behaviour was also found during the F4-flight. Another inter-

esting feature is the enhanced fluctuation at 550 Hz around the apogee of the rocket. This was only observed on the Fl-flight; its origin yet clear. If the spectral power is expressed as a power W) k takes values around

about trace is not law

= Qf”

-0.6

in the height range where

Table 2. Technical details of the modified RPA 1.

Sensor

Weight Material Grid material Grid transparency Opening angle

Dimension : 83 mm 0 x 70 mm 2 x 320 g Aluminum, gold plated Tungsten, gold plated 0.68 28

II.

Elecfrometer

(Housed within the sensor capsule) Range 5. 10e6 to IO-” A Automatic correction for zero and gain, in-flight calibration

III.

Electronic

Dimension Weight

l05xl05xl80mm 1500 g

IV.

Miscellaneous

Power Voltage Temperature range Telemetry, scientific data Housekeeping

7.5 w 28 V (built-in d.c.d.c. 060°C 128 kbitjs 3 x O5 V analog

box

Grids GI, entrance grid G2/G3, retarding grid G4, shielding grid G5, shielding grid Collector

Density fluctuation mode +3.5 v +3.5 v +3ov +15v +3ov

converter, 25 kHz)

Electron temperature mode 3.5... -1.5v 3.5... -1.5v +3ov +15 v +3ov

Fig. 1. False colour plot of electron density fluctuation power vs frequency and flight time for the ROSE flight Ft. The flight time 100 s corresponds 85.5 km altitude, 180 s is the rocket apogee at 116 km, and 250 s corresponds to 92.7 km on the downleg.

to

719

Electron density fluctuation measurements (22.1

150

1.88

250

200 Flight

time

17:OO

UT)

300

(5)

Fig. 2. Spectral index k of the fluctuation power (P = uf”) vs flight time for the ROSE flight Fl

the strong fluctuations are present (Fig. 2). At somewhat greater heights, this exponential index approaches 0, that is, the spectra look like white noise. In Fig. 3 mean relative density fluctuations AnJn, are plotted, which were integrated over the frequency range of 30-1000 Hz. The lower limit of this integration interval was chosen to avoid a bias resulting from spin and nutation effects which are present at the lower frequencies. These plots show very clearly the limited height range in which the E-region instabilities are excited. During Fl (upper panel) this range extended from about 95 to 110 km, and during F4 (lower panel) its upper boundary was clearly above 110 km, probably due to the higher electric field in this case. The profiles for F3 (middle panel) did not show a distinct maximum but were seriously affected by the large coning from which this flight suffered. In addition the electric field was quite low at that time. The high density fluctuations around 90 km altitude on the downleg are most likely due to neutral turbulence, since it is unlikely that the E-region instabilities are so strongly excited at this altitude due to the strong collisional damping. Unfortunately, no fluctuation profiles could be derived for the F2-flight due to interference.

4.

ANGULAR DISTRIBUTION OF THE DENSITY FLUCTUATIONS

During one spin period the perpendicular looking sensor measures density fluctuations from different azimuthal directions. In order to resolve these fluctuations as a function of azimuth in the plane perpendicular to the magnetic field, 32 samples at 0.5 ms resolution were taken. This total duration of 16 ms corresponds to an azimuth angle of about 15” (flight Fl) or 16” (flight F4). Each set of 32 samples was

Fourier-analyzed after trend removal and the spectrum was integrated in order to obtain one power average for the frequency range 62.5-1000 Hz. A total of 125 of these averaged power values, corresponding to 2 s (i.e. about 3.6 spin periods) are combined in a directional histogram. Figure 4 shows three of these histograms at different flight times for the flight Fl. The arrow denotes the direction of the E x B drift as seen from the moving rocket payload. The shaded area characterizes the wake region of the sensor. It is obvious that during the upleg the greatest fluctuation power levels are measured when the sensor was looking into the E x B drift direction. The same was still true around the rocket apogee (middle panel), although the fluctuation power was quite low here, because the rocket had already left the height region where the E-region instabilities are most strongly excited. During the downleg the angular distribution looked quite different than during the upleg. It is very broad and apparently shifted with respect to the E x B drift direction. It should also be noted that the power values are about twice as large as during the upleg. The corresponding histograms for flight F4 show qualitatively the same pattern (Fig. 5). The shift of the histogram with respect to the Ex B drift is seen here already around the rocket apogee and particularly during the downleg. The direction of the E x B drift was in both cases derived from data measured by the electric field probe (RINNERT, 1991, this issue). For the flight F3 such an analysis was not attempted due to the strong coning of the rocket. 5. ELECTRON TEMPERATURES As already mentioned in Section 2, the RPA was operated in the temperature mode every 3 s for

720

K.

ROSE-FLIGHT

1

SCHLEGEL

(22.11.86

UPLEG

120

1700

UT) DOWNLEG

120 _

110 f d r' .P I"

100

90

ROSE-FLIGHT 130

t r' .F 8

z-

3

(7.2.89

UPLEG

23.36

UT) DOWNLEG

130 _

r

120

120

110

110

100

100

90

90

90 i 0

12

3 6nJn.

4

5

Boo-

5

Wn.

@I

ROSE-FLIGHT

4

(9.2.89

23.42

(%I

UT)

UPLEG

130

kg>

ijs

90 _

90 _ 90.

1

1

1

1

I

I

80 0

012345 Win.

(V

I

12

I 3-4

Wn.

I

, 5

@I

Fig. 3. Mean relative density fluctuations An&, averaged over the frequency range 3&1000 Hz for the ROSE flights Fl, F3 and F4.

721

Electron density fluctuation measurements ROSE-FLIGHT

t =

130.5

132.6

s

t =

1

(26.11.88

168.3

1000

1000

800

800

1700

170.4

UT)

s

t = 218.7

220.7 s

1600 1400 1200

Fig. 4. Angular distribution of the density fluctuation power for different flight times during the ROSE flight Fl. The arrow shows the direction of the E x B drift ; the shaded area characterizes the wake region of the sensor.

ROSE-FLIGHT t =

124.0

126 0 s

4

t =

(9.2.89

164.5

2342

166.6

UT)

s

t = 242.2

244.2

s

Fig. 5. Same as Fig. 4, but for the ROSE flight F4.

100 ms. The usual data evaluating 1978), that is, the formula

theory

(SPENNER,

electron densities prevailing in the E-region. In the Fregion and the topside ionosphere this method yields very reliable electron temperature data (SPENNERand PLUGGE, 1978 ; WILHELMet al., 1987).

(U, = retarding potential, k = Boltzmann’s constant, jij, = measured current ratio) yielded quite unrealistic temperature estimates, always well above 1000 IL That is about a factor of 334 higher than the electron temperatures measured with the incoherent scatter facility EISCAT (KOHL, 1991, this issue) dur-

A complete discussion of the RPA results in terms of the respective plasma instabilities will be given in a separate paper together with the results provided by the other experiments of the ROSE flights (ROSE et

ing the flights. Our conclusion from these unrealistic electron temperatures is that the above-mentioned theory probably has to be modified with respect to the high electron-neutral collision frequencies and low

al., 1991). In the following we just comment upon our results and compare them with the results of similar earlier studies. The relative density fluctuations measured during

6.

DISCUSSION

722

K. SCHLEGEL

the ROSE campaign are in general agreement with results published by many other authors: An&r= values of several per cent but generally less than 10% were obtained by KELLEYand MOZER(1973), OGAWA et al. ( 1976), THRANEand GRANDAL(198 1) and PFAFF et al. (1984). The latter investigations have all been performed in the aurora1 zone, but similar average density fluctuations are reported from the equatorial electrojet (e.g. PRAKASHet al., 1972; PFAFF er al., 1987a,b). The fact that the strongest fluctuations occur at frequencies below about 200 Hz, clearly visible in Fig. 1, has also been reported by several authors (PFAFFPE ul., 1984; LABELLEet al., 1986; PFAFFet al., 1987a,b). PFAFF ef al. (1985) attributed these strong echoes which tend to occur at somewhat lower altitudes than the higher frequency echoes to the gradient drift instability, since they simultaneously observed strong electron density gradients at these altitudes. The spectral index seems to vary quite strongly in the different experiments. Values of k ranging from almost zero (THRANEand GRANDAL, 1981) to - 1.5 (OGAWA ef nl., 1976; PRAKASHet al., 1972) have been reported. The actual value is apparently strongly dependent on the altitude where the spectrum is recorded, and on the relative importance of the gradient drift to two stream irregularities. Our values frt perfectly into such a pattern. Angular distributions of electron density fluetuations have so far not been reported in the literature, only the corresponding distributions of electric field fluctuations (e.g. PECSELIet at., 1989). During the upleg our results show the strongest fluctuations in the E x B direction as expected, but a quite different behaviour during the downleg. Several possibilities to explain the disagreement between the E x B drift direction and the direction of maximal fluctuations can be quoted. If these strong ~uctuations stem from gradient drift waves, the direction ~rpendicular to the gradient (which is not known in our case) may be

more important than the direction of the E x B drift. Neutral turbulence could also play a role here. These two possibilities do not explain, however, that the differences occur only during the downleg. The most probable reason may be some local disturbance from the falling payload. The RPA sensors are mounted on the top; thus during the downieg they measure in a medium which is not directly in the wake of the payload but which is probably disturbed by the turning of several long booms with massive objects (see ROSE et al., 1990, for the payload configuration). If this -. explanation is true, the values of An&z@measured on the downleg may be questionable. It should also be mentioned in this context that we presented data from the side-looking sensor only throughout this paper. The front-looking sensor showed generally similar results but with somewhat lower AnJn, values. This can perhaps be explained by the effect of a shock just in front of this sensor. Correlation studies of the results from both sensors are under way in order to obtain information about the phase velocity of the irregularities. Such a technique was applied by LABELLEet al. (i 986). A mystery remains the distinct wave feature at about 550 Hz between 170 and 190 s clearly visible in Fig. 1 which occurred only during one flight. No known wave type occurring in the E-region can be attributed to such a frequency. A strong Doppler shift of a certain wave type is also unlikely since the feature occurs around apogee where the rocket moves rather slowly. This spectral enhancement perhaps has no relation to the plasma instabilities studied here, since it occurs clearly outside of the height region where their effect is strongest. Acknowledgements-We gratefully acknowledge the help of Mr E. Prager during the payload preparation and during the launches. To Mr W. Noack we are indebted for the excellent electronic design of the RPA. The ROSE project was funded by a grant from the Bundesmlnlste~um fiir Forschung und Technologie, E.R.G.

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1990

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1978 1987

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723

Reference is also made to the following unpublished material: ROSEG., RINNERTK., SCHLEGEL K., NESKEE., FR~KERA., L~~BKEN F.-J., STEINWEG A., KRANKOWSKYD., L~~MMERZAHL P., MAUERSBERGER K., ANWE~LER B., L&m H., OELSCHL~~GEL W., DEHMELG., WARNECKEJ., KOHL H. and NIELSENE. SPENNER K. WILHELMK.. R~NNERTK.. SCHLEGEL K.. KOHL fi., KL~CKERk., LUHR H., OELSCHLKGEL W., D~~MELG., GOUGHM. P., HOLBACKB. and OYAMA K.-I.

Katlenburg-

Katlenburg-